This invention relates to the production of copper. In one aspect, the invention relates to the pyrometallurgical production of copper while in another aspect, the invention relates to the pyrometallurgical production of copper using a continuous converting furnace. In yet another aspect, the invention relates to the pyrometallurgical production of copper using a continuous flash converting furnace equipped with a forebay.
The production of copper is ancient. Starting with finds of copper metal that were virtually ready for fabrication into various tools, man has learned over the millennia to recover essentially pure copper from ever more dilute ores (e.g. 0.2% or less copper). The two principal forms of copper production are pyrometallurgical and hydrometallurgical, the former the subject of this invention.
The pyrometallurgical production of copper is a series of multistep concentration, smelting, and refining procedures. Typically starting with an ore comprising one or more of a copper sulfide or copper-iron-sulfide mineral such as chalcocite, chalcopyrite and bornite, the ore is converted to a concentrate containing usually between 25 and 35 weight percent (wt %) copper. The concentrate is then converted with heat and oxygen first to a matte (typically containing between 35 and 75 wt % copper), and then to blister copper (typically containing at least 98 wt % copper). The blister copper is then refined, usually first pyrometallurgically and then electrolytically, to copper containing less than 20 parts per million (ppm) impurities (sulfur plus noncopper metals, but not including oxygen).
The conversion of copper concentrate to blister copper with heat and oxygen is known generally as smelting, and it comprises two basic steps. First, the concentrate is "smelted" to copper matte and second, the matte is converted to blister copper. Typically these steps are performed in separate furnaces, and these furnaces can vary in design. With respect to the first step, i.e. the smelting step, solid copper concentrates are introduced into a smelting furnace of any conventional design, preferably a flash smelting furnace, which is fired by the introduction of fuel and air and/or oxygen through a burner, and from which slag is tapped periodically and off-gases are routed to waste handling. In a flash smelting furnace, the copper concentrates are blown into the furnace through a burner together with the oxygen-enriched air. The copper concentrates are thus partially oxidized and melted due to the heat generated by the oxidation of the sulfur and iron values in the concentrates so that a liquid or molten bath of matte and slag is formed and collected in the basin (also known as the "settler") of the furnace. The matte contains copper sulfide and iron sulfide as its principal constituents, and it has a high specific gravity relative to the slag. The slag, on the other hand, is composed of gangue mineral, flux, iron oxides and the like, and it has a low specific gravity relative to, and thus floats on top of, the matte.
The molten copper matte and slag are separated in any conventional manner, typically by skimming the molten slag from the matte through tap holes in the furnace walls. The slag tapholes are located at an elevation on the furnace walls that allows slag withdrawal from the furnace without removal of molten matte. Tapholes for the molten matte are located at a lower elevation on the furnace walls that allows the withdrawal of molten matte without the withdrawal of slag. The molten copper matte is then either transferred directly or indirectly (e.g. by way of a holding furnace) to the converting furnace by any conventional means, e.g. launder or ladle, or its converted to solid form, e.g. granulated, for storage and later use as a feed to a converting furnace.
Converting furnaces are basically of two types, flash (also known as suspension) and bath, and the purpose of both furnaces is to oxidize, i.e. convert, the metal sulfides to metal or metal oxides. Representative bath furnaces include those used by Noranda Inc. at its Horne, Canada facility, by Mitsubishi Materials Corporation at its Naoshima, Japan facility, and by Inco Limited at its Sudbury, Canada facility. Representative flash converting furnaces include that used by Kennecott Utah Copper Corporation at its Magna, Utah facility.
Regardless of its design, the converting furnace contains a bath of molten blister copper which was formed by the oxidation of copper matte that was fed earlier by one means or another to the furnace. The bath typically comprises blister copper of about 50 centimeters in depth upon which floats a layer of slag of about 30 centimeters in thickness. If the furnace is a rotary bath-type, then the molten metal and slag, separately of course, are poured from a mouth or spout on an intermittent basis. If the furnace is stationary, then outlets are provided for the removal of both the slag and blister copper. These outlets include tapholes located at varying elevations on one or more of the furnace walls and in a manner similar to that used with the smelting furnace, each is removed from the furnace independent of the other.
Alternatively, the bath contents (i.e. the metallurgical melt) of the converting furnace is removed through a forebay or syphon which is attached to the furnace. The forebay is in open communication with the settler of the furnace by a passageway that allows for the continuous removal of both slag and blister copper. The slag and blister copper maintain their phase-separated relationship as they enter the forebay.
The forebay comprises a slag skimming chamber or zone equipped with a weir on one end and at least one tapping or overflow notch on at least one sidewall. The notch or notches is or are located at an elevation on the sidewall such that only slag enters and is removed from the forebay. The bottom of the notch(es) is(are) above the top surface of the metal product.
The weir of the forebay is located downstream from the slag overflow notch, and it is positioned (usually attached to both forebay side walls) such that it acts as a dam to the slag but not the metal product which underflows the weir to a point beyond the weir in the forebay referred to as the riser chamber or zone. The metal overflows this riser chamber through a metal overflow notch(es) on the end and/or side walls. In this manner, the molten metal product continuously overflows the end wall of the forebay into any means, e.g. a launder, tundish, etc. for transfer to another vessel (e.g. a holding furnace, an anode furnace, etc.).
Unlike a forebay, only blister copper enters a syphon. The opening between the syphon and the settler zone of the furnace is sized and positioned such that only blister copper has access to the syphon, i.e. the opening is positioned below the bottom surface of the slag layer. In this manner, the settler endwall acts as a weir relative to the slag gaining entry to the syphon. In these types of arrangements, the slag is removed through tapholes in the settler side or end walls.
The physical and chemical separation that occurs between the slag and blister copper is not complete and as such, the slag contains copper (usually in the form of cuprous oxide, i.e. Cu.sub.2 O, and copper metal, i.e. Cu.sup.0) and the blister copper contains various waste and unrecovered mineral values, e.g. sulfur (principally in the form of cuprous sulfide, i.e. Cu.sub.2 S), ferrosilicates, cuprous oxide, etc. The copper in the slag is potentially lost metal value which is recovered by recycling the slag back to the smelting furnace. The waste and unrecovered mineral values in the blister copper are impurities which are eventually removed either in the anode furnace or through electrorefining.
The oxidation of copper sulfide at the interface of the slag and blister copper phases is known. However, the beneficial effect of this oxidation is minimized, particularly in stationary furnaces, by the relative quiescent state of the interface (because the activities of reacting sulfur and oxygen species must be high enough to produce sulfur dioxide at a pressure greater than that superimposed on the interface by the gas pressure in the furnace (about 1 atmosphere absolute) and the layer of slag above the interface (about 0.1 atmosphere absolute)). The oxidation will also be limited by the time in which the interface exists before the slag and blister copper are separated into different fractions.
Once the blister copper is separated physically from the slag, typically it is transferred by any suitable means, e.g. launder, ladle, etc., to an anode furnace for further pyrometallurgical refining (although in some instances, it may be transferred first to a holding furnace). Anode furnaces (not shown) are generally constructed as cylindrical vessels mounted on girth gear that enable them to rotate. They are generally equipped with a mouth to feed material, a burner to heat the contents, and tuyeres to feed gases into the metal bath. Tuyeres consist of pipes that pass through the vessel shell connected to supplies of inert, oxidizing, and reducing gases.. Blister copper in conventional operation is batch fed from ladles through the mouth of the vessel until a complete charge has been accumulated over a period of hours. During this time the burner is lit and maintains the charge in a molten condition.
Upon achieving a full charge that may weigh typically one hundred to six hundred tons, depending on the size of the furnace, the vessel is rotated one way into position so that the tuyeres are submerged beneath the metal surface and a sequence of gases are blown into the metal. Tuyeres may number typically between one and four depending on the size of the vessel.
The first sequence of gas blowing is termed the oxidation blow, consisting of the passage of mixtures of inert gas, air and oxygen into the blister copper to lower its sulfur content. The actual composition and volume of gases blown in this sequence is variable within limits and determined by the particular composition of the blister copper and the heat balance of the blowing operation. The desulfurizing operation is exothermic and the build-up of heat in the furnace can be controlled by varying the gas flow, and its inert (typically nitrogen) and oxygen content. In the process of this oxidation, slag is generated consisting of the remnants of iron, silica and other impurities from the prior smelting and converting processes. In some anode furnace sequences, the oxidation blow is usually split into two distinct steps separated by a slag removal stage. Slag is removed by turning the vessel back to its initial position, then continuing the rotation to the opposite side so that the mouth on the shell is low enough for slag to be poured off the surface of the metal into a suitable container. This collected slag is returned to the upstream process for valuable metals recovery. The furnace is then returned to its blowing position for further oxidation and removal of sulfur.
The sulfur is removed from the metal during the first sequence, or oxidation blow, as sulfur dioxide gas that evolves from the metal bath with unreacted oxygen and inert gases. The composition of this gas is low in sulfur dioxide, being typically 5,000 ppm during the initial blow when sulfur content is at a maximum, and dropping to less than 500 ppm when almost all of the sulfur has been removed. This gas is unsuitable for recovery of sulfuric acid and is neutralized and captured in gas scrubbing equipment.
The second sequence of gas blowing is called the reduction blow, consisting of the passage of inert and reducing gases (such as ammonia or natural gas/steam) into the desulfurized copper to reduce its oxygen content and form anode copper. The actual volume and composition of the gases blown during this sequence is again variable within limits, and determined by heat transfer and mass transfer considerations.
The conventional anode refining operation described in the foregoing paragraphs has the following disadvantages:
1. The operation is batch, with several stages that involve careful control and operator involvement. PA1 2. In a continuous converting operation, the conventional batch anode refining operation introduces a potential bottleneck and can disrupt optimum converter operation. PA1 3. The variable exhaust volume from the batch refining operation requires a gas system capable of a higher-than-average gas flow with consequent higher capital charges and operating costs. PA1 4. The accumulation of blister copper at the commencement of the refining cycle, and the reheating of refined charges at the end of the refining cycle requires a high capacity oxygen-enriched burner for rapid heat input. The high temperature flame increases wear on the anode furnace refractory and produces a high thermal load on the gas handling system. PA1 5. The inevitable variation in gas volumes introduced into the melt within the anode furnace during the different sequences of operation increases the potential for furnace refractory wear around the tuyere mouths. This leads to shutdowns to repair the refractory and the need for spare capacity in the form of additional anode furnaces that are expensive on capital and operating costs. PA1 6. The need for multiple anode furnaces as a result of batch operation and intermittent maintenance adds to the complexity of mechanical and control systems. PA1 1. The anode refining furnace performs a continuous refining operation on a continuous stream of molten copper received directly from a continuous converting furnace or via an intermediate holding furnace. The blister copper enters at one end of the furnace and exits as refined anode copper at, or towards, the other end. PA1 2. The superheat present in the continuous blister stream is utilized directly in the refining operation rather than be dissipated in the batch collection stage. PA1 3. The residual sulfur in the blister copper stream is not removed in a separate oxidative stage but is removed to a degree determined by the initial oxygen content of the blister copper. PA1 4. The level of sulfur in blister copper suitable for continuous refining is obtained as a natural feature of flash converting operation or as a result of subsequent additional removal in the continuous tapping device or intermediate holding furnace. PA1 5. The level of oxygen in blister copper suitable for continuous refining is obtained as a natural feature of continuous converter operation, or if insufficient, is added in the form of a solid, oxygen-donating compound such as copper oxide or is added as gaseous oxygen. This insufficiency is corrected by addition to the stream leaving the continuous converter; while it is in transit to the anode furnace or holding furnace; while in the holding furnace; or while in the anode furnace; or by combinations of these methods. PA1 6. The essentially continuous sulfur-bearing off-gas from the above continuous refining operation is beneficially routed to the process gas stream of the continuous converter, or associated smelting process. The majority of the sulfur dioxide is recovered as sulfuric acid. PA1 7. Any tendency to form a copper oxide slag in the continuous refining operation is reduced by the presence of sulfur in the incoming feed. Any such slag formed in the furnace is re-mixed with the high sulfur blister at the feed end of the furnace to utilize the oxygen content of the slag. PA1 8. The slag layer untimately formed on the melt being refined in the anode furnace is removed continuously or semi-continuously. Residual gangue in the incoming blister for example silica, lime, iron and alumina, together with some copper oxide and minor elements such as lead, bismuth and antimony, comprise the slag phase. PA1 9. The slag properties are controlled by the optional addition of fluxing agents in any suitable manner, e.g. injection. The thickness of slag on the refining melt is controlled by the position of the slag removal device, such as notch, tap hole, underflow, according to known principles. PA1 A. Feeding copper matte to the furnace, the furnace operated at conditions sufficient to convert the matte into molten blister copper and molten slag; PA1 B. Converting within the furnace the matte to molten blister copper and molten slag; PA1 C. Collecting the molten blister copper and the molten slag in the settler zone of the furnace such that the slag contains an amount of copper oxides and copper metal and floats upon and forms an interface with the molten blister copper, and the blister copper contains sulfur in excess of about 500 ppm; PA1 D. Agitating the blister copper/slag interface with the blister copper/slag interface agitation means such that the sulfur content of the blister copper is reduced to less than about 500 ppm and the amount of copper oxides and copper metal in the slag is also reduced; and PA1 E. Removing the molten blister copper with the reduced sulfur content from the furnace. PA1 A. A continuous copper converting furnace for producing blister copper containing less than about 700 ppm sulfur and less than about 7000 ppm oxygen, the furnace having a (i) settler zone, (ii) molten blister copper/molten slag interface agitation means, and (iii) a forebay in open communication with the settler zone; PA1 B. An anode furnace having blister copper reducing means for reducing the oxygen content of the blister copper produced in the continuous copper converting furnace to less than about 7000 ppm; and PA1 C. Blister copper transfer means for transferring the blister copper containing less than about 700 ppm sulfur from the forebay or tapping device of the continuous copper converting furnace to the anode furnace.
By contrast with the shortcomings and limitations of conventional anode refining described above, this invention combines continuous converter operation with continuous refining furnace operation in the following ways and with the following advantages:
After the reduction step, the melt (i.e. anode copper) is cast into anodes for electrolytic refining to cathode copper (which typically contains less than about 20 ppm total impurities, e.g. sulfur, oxygen, arsenic, bismuth, antimony, silver, etc.).
While the present method of producing anode copper has evolved to a high state of both economic and environmental efficiency, improving operating efficiency is an eternal quest. One area of operation that lends itself to improvement is the operation of the anode furnace, specifically elimination of the oxidation stage. With the elimination of this stage, the throughput of the anode furnace can be significantly increased without any changes to the furnace itself. However to achieve this efficiency, the blister copper that is delivered to the anode furnace should ideally have less than about 500 ppm sulfur and less than about 4500 ppm oxygen. This in turn requires operating the upstream equipment, particularly the converting furnace in a manner that produces blister copper with sulfur and oxygen contents less than these numbers.